(a) Field of Invention
This invention relates to cryogenic apparatus for freezing a liquid product into pellets, and more particularly to cryogenic apparatus having a controlled dwell time such that the product to be frozen is in contact with the freezant for a controlled period of time. Additionally, the invention provides a more efficient means of using a freezant such that less of the freezant is lost during the process.
(b) Background of Invention
It has long been known that in order to deep freeze something quickly it could be introduced into a coolant such as liquid nitrogen, which is typically at a temperature of around minus 300 degrees F. There are a number of problems, however, associated with introducing a liquid product into the liquid nitrogen for freezing.
The product that ultimately emerges is frozen very quickly to a very low temperature, depending on the dwell time--the time that it remains in the liquid nitrogen. It is desirable that the temperature of the frozen product that is removed from the liquid nitrogen be fairly constant such that no more heat than necessary is absorbed into the liquid nitrogen. This is necessary for the sake of efficiency, since liquid nitrogen is very expensive. In many types of cryogenic apparatus, the product becomes suspended in liquid nitrogen in a relatively uncontrolled manner. The dwell time of the various pieces of product can vary a great deal. This can cause a temperature difference of up to 100.degree. F. between different pieces of the product at the time they are removed from the liquid nitrogen.
Once the liquid product is frozen to a very low temperature, however, it is quite acceptable to have the frozen product to come up to a higher temperature as long as the product remains frozen.
It is also generally desirable that the frozen product be of a fairly consistent size when it is frozen. This is for two reasons. Firstly, a relatively consistent size product is produced and it is easier to work with. Working with a relatively consistent size product is highly advantageous in various industries no matter what the type of frozen end product being used. Industries such as bakeries, drug manufacturers, processed food producers, and the like, need to measure fairly exact quantities of frozen product. This is more easily done if the pellets of frozen product are of relatively consistent size, especially where automated measuring machines are used. More exact measurement of frozen product provides for a more controlled means of producing a more consistent quality end product. Furthermore, a more consistent size frozen product will thaw at a more consistent rate, thus producing a better quality end product. Secondly, as mentioned above, this allows all of the pellets or particles to be frozen about the same amount. If particles are indeed smaller, they will be frozen more deeply, which is a waste of liquid nitrogen.
Another problem with entering liquid product into a bath of liquid nitrogen is that the liquid nitrogen typically boils fairly violently at the surface. This means there is a fair degree of bubbling at the surface which would tend to splash the liquid nitrogen at the liquid product as it is being dropped into the liquid nitrogen. This tends to break up the product and thus small droplets of varying size of the liquid product would be entering the liquid nitrogen, and subsequently different size pellets would be formed. Furthermore, it would not be possible to have a nozzle immediately above the surface of the nitrogen since the low temperature of the nitrogen would freeze the product as it was coming out of the nozzle, thereby plugging up the nozzle. The nozzle would, therefore, have to be a suitable height above the surface.
If the product was pumped or ejected out of the nozzle with sufficient force so as to preclude the product from freezing in the nozzle, then it would be possible to have the nozzles closer to the surface of the liquid nitrogen.
Typically, it has also been found that creating movement or flow within the liquid nitrogen, precludes a large degree of the surface bubbling that is typically encountered. Such movement can be caused by impellers, paddles, or jets.
One way to improve the efficiency of a cryogenic apparatus such that the dwell time is fairly constant, thereby making the whole process more efficient in terms of not wasting liquid nitrogen, is to introduce the product into a moving volume of liquid nitrogen for a controlled period of time. This can be accomplished by having liquid nitrogen flow at a relatively constant rate along a downwardly sloping ramp or sluiceway, where it can flow until it reaches a reservoir. The amount of time taken for the liquid nitrogen to travel the ramp or sluiceway is fairly constant, and controllable depending on the length and slope of the ramp or sluiceway. It is, therefore, possible to control the dwell time of the liquid product in the nitrogen by introducing the product onto the sluiceway at a given point, and removing the frozen product at a given point.
Introduction of the product is done basically in the same manner as introducing such product to a reservoir of liquid nitrogen for freezing--the product is dropped from one or more nozzles. The movement of the liquid nitrogen flow on the ramp or sluiceway largely precludes the boiling action of the liquid nitrogen that typically interferes with proper operation of the nozzle or nozzles.
More importantly, it is possible to channel the flow of liquid nitrogen, since it is in a ramp or sluiceway, into a means for separating the frozen product from the liquid nitrogen. Such means can be a fairly simple screen that catches the product, removes it from the flow of liquid nitrogen and transports it to a holding bin. The liquid nitrogen would obviously pass through the screen, and ultimately be returned to the reservoir of liquid nitrogen.
There are problems associated with the apparatus as described above in that there is a greater amount of liquid nitrogen exposed to the air than necessary, which allows for greater evaporation of the liquid nitrogen. Furthermore, the movement and general agitation of the liquid nitrogen will also cause greater evaporation. Since liquid nitrogen is quite expensive, it is undesirable to have any more evaporation of liquid nitrogen than is necessary.